This invention is in the field of cold chamber die casting machines. More particularly, the invention relates to an injection piston which provides improved injection, lubrication and cleaning of the injection sleeve.
The injection piston is comprised of a plunger tip, plunger tip ring, a cap to retain the plunger piston ring on the plunger tip, a lubricating chamber and a scraper and guide ring. The cap, plunger piston ring and scraper and guide ring are fastened to the plunger tip. An annular arcuate recess about the circumference of the plunger tip in combination with a series of tilted and radial lubrication nozzles form a lubrication chamber within the injection sleeve. The extent of the lubrication chamber enables a substantial portion of injection sleeve to be directly lubricated before withdrawal of the plunger tip in the injection sleeve in preparation for the filling cycle.
In cold chamber die casting, the injection piston is located within the injection sleeve of the cold chamber die casting unit. The injection piston is connected by a connecting rod to an injection piston rod to an injection unit piston. The withdrawal of the injection unit piston results in the withdrawal of the injection piston within the injection sleeve to a fill position. In the fill position molten metal is poured into the space in the injection sleeve above the injection piston. Once the dies of the cold chamber die casting machine are closed and clamped, the injection cycle is commenced. In the injection cycle, the injection unit piston drives the piston rod, connection rod and injection piston upwardly within the injection sleeve transporting the molten metal in the injection sleeve into the runners and die cavities. As soon as the molten metal in the dies is firm, the injection unit piston withdraws the injection piston to the fill position within the injection sleeve in position for commencement of the subsequent cycle.
One problem associated with cold chamber die casting machines is that during the injection cycle small amounts of molten metal escape between the inside of the injection sleeve and the injection piston or through a piston ring and form scrap on the interior of the injection sleeve. The problem results from the inside diameter of the injection sleeve expanding and contracting because of thermal expansion caused by receipt of molten metal followed by relative cooling during the injection cycle when the molten metal is removed from the injection sleeve. The injection plunger is also subject to expansion and contraction. Piston rings are also subject to thermal expansion and contraction which may result in a gap through a split ring or rings for the molten metal. It is important that scrap formed from metal be removed from the interior of the injection sleeve to prevent scoring of the injection sleeve which aggravates the problem. Scrap not removed when the injection piston is withdrawn from the interior of the injection sleeve may be removed in the injection cycle and enclosed in a casting resulting in a possible reject.
Another problem associated with cold chamber die casting machines is that the injection piston or the piston ring of the injection piston must be in sliding contact with the surface of the injection sleeve to prevent some molten metal under pressure from escaping between the injection piston and the injection sleeve. The injection piston contacts the injection sleeve during the withdrawal stroke as well as the injection stroke. It is necessary to lubricate the injection piston to prevent wear and lessen scoring by contact movement of the injection piston on the surface of the injection sleeve.
U.S. Pat. No. 5,076,343 discloses a die cast plunger lubrication system. The plunger tip includes a lube groove through which lubrication is forced out on the forward stroke. The disclosure states that the lubricant may be output to the outer surface of the plunger rod instead of through a lube groove. U.S. Pat. No. 4,420,028 discloses an orifice located adjacent to the piston head.
In both the above inventions there is a substantial area of the plunger tip or piston head in contact with the interior of the sleeve. In both patents the lube groove or lube orifice is very small in comparison to the length of the plunger tip.
The plunger tip of the instant invention does not contact the surface of the injection sleeve. The plunger piston ring which is located in an annular recess on the front outside surface of the plunger tip is the first part of the injection piston in permanent contact with the interior of the injection sleeve, the second part is a scraper and guide ring located in an annular recess on the rear side of the plunger tip. The plunger piston ring is retained in the annular recess on the plunger tip by a cap in the form of a disc fastened to the face of the plunger tip. The contact surface between the surface of injection piston and the surface of the injection sleeve is the outer surface of the plunger piston ring. The contact surface of the plunger piston ring is substantially less than that of the contact surface between the plunger or plunger tips disclosed in the above patent. The lubrication chamber and associated annular radial and tilted pressurized air and lubrication nozzles apply pressurized air and lubrication directly to a substantial portion of the injection sleeve initiated upon withdrawal of the injection piston.
Japanese Patent 8,068,257 discloses the use of a series of split rings located side by side on a plunger tip to decrease the surface to surface contact between the injection plunger and injection sleeve. The plunger piston ring of the instant invention does not provide a continuous passage through the ring as does a split ring. The plunger piston ring of this invention is comprised of a ring of tool steel in which a series of parallel alternately disposed inclined slots are cut alternately in the front side and rear side of a ring of tool steel. The inclined slots proceed two thirds to three quarters of the distance through the plunger piston ring. The parallel alternate inclined slots result in a plunger piston ring which is flexible without providing any opening extending completely through the plunger piston ring. The plunger piston ring acts as a guide for the plunger tip which is not in contact with the inside of the injection sleeve. The surface area of the plunger piston ring in contact with the surface of the injection sleeve in less than the surface contact of plunger, plunger tips, combined plunger tips and rings or series of plunger split rings used in combination disclosed in the prior art. The lesser surface area contact results in less metal to metal contact between the injection piston and the injection sleeve during each cycle.